goma: use GOMA_SERVICE_ACCOUNT_JSON_FILE

scripts/slave/recipes/chromium.expected/full_chromium_webkit_Android_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "Android Builder",
@@ skipping 286 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
+      "--goma-service-account-json-file",
+      "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[BUILD]/goma",
       "--",
       "blink_heap_unittests_apk",
       "blink_tests",
       "webkit_unit_tests_apk"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s"
@@ skipping 55 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@WebKit Android (Nexus4)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]